1. Field of the Invention
This invention relates to apparatus used in the processing of semiconductor wafers. More particularly, it relates to apparatus for transporting cassettes holding multiple semiconductor wafers that can be used in the automated processing of semiconductor wafers through a sequence of operations in which the wafer cassettes must be transported from one processing station to another.
2. Description of the Prior Art
The handling and storage of thin, delicate wafers for the semiconductor industry present difficult problems because of the high value of such wafers and their fragility. The wafers may be of very brittle material, such as silcon, and are usually three to six inches in diameter but only a few thousandths of an inch thick.
The processing of such wafers usually requires them to be successively bathed in and drained free of liquids, sometimes at elevated temperatures. They may also be dried, placed in an epitaxial reactor or diffusion furnace or merely transported from one location to another. In the past few years it has become common to use cassettes in the form of small baskets or racks of a size suitable to accommodate an array of wafers of a particular size to carry or store the batch of wafers during processing or transport. During processing, the cassettes together with their wafers are usually successively immersed in baths of liquids or used at other wafer batch processing stations performing other operations previously mentioned.
Several factors make it desirable to provide automation in the processing of cassettes of semiconductor wafers. First, the wafers must be processed in extremely, dust-free environments in order to achieve high yields. The hair, skin and clothing of human beings necessarily release particulate matter that can cause problems at the microscopic level in semiconductors. Second, the processing of semiconductor wafers sometimes involves hazardous or unpleasant chemicals or other environments to which human beings should not be exposed for prolonged periods. Third, the timing of certain processing operations is critical. Automated equipment is superior to human labor for repetitive sequences of time-critical operations. However, even automated equipment must be carefully designed to represent an improvement over human labor. For example, parts that move against each other can be a source of dust or particulate matter. It is also desirable to avoid direct automated or manual contact with the cassettes themselves, as this can lead to chipping, breaking or contamination of the semiconductor wafers in the cassettes.
While the semiconductor industry has developed certain standards relating to the dimensions, wafer capacity and other characteristics of the cassettes used to hold semiconductor wafers, present cassette designs (e.g., U.S. Pat. Nos. 3,923,156 and 3,961,877) do not lend themselves easily to interfacing with robotics equipment. Robotic handling equipment working with the delicacy and dexterity of the human hand is now possible in some circumstances, but such equipment is extremely expensive. Accordingly, what has been needed in the prior art is apparatus that aids in the use of automated equipment for handling cassettes that hold semiconductor wafers (or other similar semiconductor materials) during their processing. Such equipment will have the widest usefulness when it can interface with relatively inexpensive robotic devices and when it can serve as the interface for all manual or automated handling of cassettes, so that direct manual or automated handling of cassettes (which can lead to chipping, breaking or contamination of wafers) is avoided.